Search Results (52)

Coastal lagoons are dynamic and highly productive systems that offer a remarkable number of ecological services and benefits for humans. However, our understanding of them is still far from adequate. The Mar Menor lagoon is an ecosystem subject to anthropogenic pressures that have worsened in recent years. These pressures include coastal works, such as dredging and sand dumping, as well as changes in agricultural regimes that have induced a process of eutrophication that set off alarms after the eutrophic crisis that occurred in 2016. Benthic organisms, and in particular mollusks, are very sensitive to environmental variations, often serving as indicators of these changes. This work analyzes the malacofauna of the Mar Menor from 1981 to 2019 in the context of the environmental changes that have occurred in it during these years. Eighty-six species have been recorded throughout our study period, and species richness, abundances, local assemblage structures, along with changes in the main environmental parameters of the water column (salinity, temperature, and chlorophyll a concentration) have been used to explain the composition of the communities of the main lagoon habitats and to detect their spatial and temporal variations. With the information provided, the complete inventory of mollusks reported in the lagoon has been updated to 126 species. The results indicate that, during these almost 40 years, the total number of species has remained relatively constant, but with a high percentage of occasional and very rare species, along with a high rate of change from one species to another over time, accompanied by variations in the abundance and dominance of some species compared to others depending on the environmental conditions and pressures that the lagoon has undergone. The high spatial and temporal heterogeneity detected is determined by the restricted connectivity with the open sea, the diversity of environments and habitats, and the changes in environmental conditions due to human actions. Full article

The Yangtze River Economic Belt in China, a major economic and ecological region, faces critical challenges in the sustainable management of dredged sand, exacerbated by illegal sand mining practices. This study advances the understanding of integrated management models for dredged sand utilization by systematically analyzing six pilot projects through field investigations and theoretical methods. It identifies three novel management models: the traditional government-led model, the integrated “operation + concession” model, and the separated “operation + concession” model. These models provide structured approaches to enhance stakeholder collaboration, streamline resource distribution, and standardize regulatory frameworks. Furthermore, this study underscores the necessity of tailored strategies to align with local conditions, enabling sustainable and resource-efficient practices. By addressing critical gaps in prior research and proposing an actionable framework, this research offers valuable insights for global efforts to mitigate the sand scarcity crisis through innovative sand management. Full article

From both economic and environmental points of view, the reuse of dredged sediments in the direct onsite casting of concrete represents a promising method for replacing sand. The aim of this study was to develop a cementitious material that (i) reuses the thin particles of sediments; (ii) has a low density due to the incorporation of air foam in the material; and (iii) achieves a minimum mechanical strength of 0.5 MPa for embankment applications. This study focused on the characterization of a non-standard “concrete”, which is a mixture of a synthetic soil (80% montmorillonite and 20% calibrated sand) and cement. To reduce its density, air foam was incorporated into the material during the manufacturing process (air-foamed lightweight clay concrete—AFLCC). The study results highlight that a density around 1.2 (unit: g/cm³/1 g/cm³) can be obtained. This density reduction can be obtained with a certain degree of workability when the material is in a fresh state. To obtain this workability, a certain amount of water must be added; however, the addition of water has a significant impact on the compressive strength of the AFLCC. As such, a mathematical equation correlating the compressive strength, the density, and the percentage of cement is proposed in this study. The mechanical strength results of the AFLCC at different times, in conjunction with the Vicat results, show that the porosity created by the air foam has the effect of slowing down the hydration mechanism of the cement. The porosities obtained are consistent with the density results. The characteristic radii indicate large pore sizes for formulations with low fluidity in the fresh state when air bubbles are incorporated. Full article

The study of dredged fill in Guangdong (GD), China, is of great significance for reclamation projects. Currently, there are relatively few studies on dredged fill in Guangdong, and there are many differences in the engineering characteristics of dredged fill foundations formed through land reclamation and natural foundations. In order to have a more comprehensive understanding of the physico-mechanical properties of blowing fill in the coastal area of GD and to understand the effect of its long-term creep row on the long-term settlement and deformation of buildings, the material properties, microstructure, elemental composition, triaxial shear properties, and triaxial creep properties of dredged fill in Guangdong were studied and analyzed through indoor geotechnical tests, scanning electron microscopy (SEM), X-ray diffraction (XRD), and conventional triaxial shear tests and triaxial creep tests. The test results showed that the Guangdong dredged fill is characterized by a high water content, high pore ratio, and high-liquid-limit clayey sand, and the mineral composition is dominated by quartz and whitmoreite. The scanning electron microscopy results showed that the particles of the dredged fill showed an agglomerated morphology, and the surface of the test soil samples had scaly fine flakes and a fragmented structure. In the triaxial shear test, the GD dredged fill showed strain hardening characteristics, and the effective stress path showed continuous loading characteristics; the consolidated undrained shear test showed that the GD dredged fill had shear expansion characteristics under low-perimeter-pressure conditions. It was found that, with an increase in bias stress, the axial strain in the consolidated undrained triaxial creep test under the same perimeter pressure conditions gradually exceeded the axial strain in the consolidated drained triaxial creep test. The results of this study are of theoretical and practical significance for further understanding the mechanical properties of silty soils in the region and for the rational selection of soil strength parameters in practical engineering design. Full article

As a strategically important resource with stable properties, high-purity quartz plays a crucial role in various fields such as high-purity quartz crucibles, semiconductors, and electronics. Currently, the availability of high-quality quartz resources is gradually diminishing, and there are no high-quality quartz deposits in some areas, making the improvement of low-quality quartz essential. This study focuses on the quartz sand produced through river dredging in Yueyang. The detected SiO₂ content is 92.31%, which enables the secondary utilization of waste resources and provides significant environmental benefits. In this study, the iron oxide removal efficiency achieved through magnetic separation pretreatment can reach 87.24%. Following flotation, calcination, and leaching processes, high-purity quartz products with a total impurity content of less than 100 ppm were successfully obtained. In addition, the factors affecting magnetic separation and calcination effect were studied, and the process parameters for reference were obtained. The mechanism of microwave impurity removal was investigated using SEM, TG-MS, and in-situ XRD, demonstrating the feasibility of producing high-purity quartz from low-quality quartz. Full article

Water jet scouring technology is extensively applied in marine engineering, harbor maintenance, river training, and various other fields, showcasing a broad spectrum of potential applications. However, achieving a comprehensive understanding of the transient sand scouring characteristics of water jets remains challenging due to the inherent complexity of the coupled flow structure involving submerged jets and environmental fluids, along with the intricate dynamics of two-phase flow. This study, rooted in numerical simulation and experimental validation, introduces pulse characteristics into a submerged jet. A thorough investigation is conducted to explore the transient sand scouring characteristics and sand transport laws of the submerged jet under diverse working conditions. The results of this study revealed that the main reason for the asymmetry of the sand pit morphology is not the non-uniform distribution of sand grains, but more likely caused by turbulence effects. Simultaneously, within the initial 0.25 s of the pulse cycle, suspended sediment resulting from the pulsed jet in the preceding cycle gradually transports to the dune and its surrounding areas. Subsequently, from 0.25 s to 0.5 s, sediment on both sides of the pit’s bottom undergoes movement and amalgamation with the sediment that remained unsettled during the previous cycle. The findings reveal that higher jet velocities significantly enhance sediment suspension, migration, and redeposition, leading to deeper erosion and the rapid formation of the sand pit’s outline within 2 s. Additionally, the jet velocity and the impact distance are identified as critical factors influencing erosion depth and sediment dynamics. These insights advance the understanding of erosion mechanisms driven by pulsed jets, highlighting their impact on sediment transport processes. The research findings provide important guidance for dredging and ocean engineering fields and offer a theoretical basis for improving the understanding of submerged jet scouring mechanisms. Full article

Silt removal is crucial for maintaining navigable waterways in harbors. Jet pumps, without moving parts, are highly suitable for underwater operations such as channel dredging in port environments. Despite their structural advantages in slurry handling, the prolonged transport of solid–liquid two-phase flows can lead to wear on the wall materials, resulting in decreased efficiency and potential pump failure. The wear characteristics of the jet pump walls due to sand particles of varying grain sizes were experimentally investigated. The characteristic of the sands having a higher distribution above the axis as they enter the jet pump was captured by a high-speed camera. The experiment recorded the variations in mass loss at different sections of the jet pump over a period of 120 h, identifying that backflow within the throat region is a significant contributor to wall wear. Scanning electron microscopy was employed to examine the microstructure of the abraded pump surfaces. It was found that there are noticeable differences in the surface wear microstructure across various pump areas, and that particles of different grain sizes result in distinct wear patterns on the pump surfaces. The underlying causes of this phenomenon were discussed from the perspective of particle motion. Full article

High-intensity anthropogenic activities have greatly altered the estuarine-shelf depositional processes of sediments, and the intensity and frequency of the impacts of human interventions have far exceeded the natural development of estuarine systems. Since the reform and opening up, human activities such as dams, sand mining, channel dredging, and reclamation have already caused anomalous changes in the dynamical–sedimentary–geomorphological processes of the Lingdingyang Estuary (LE). Analyzing the impact of high-intensity anthropogenic activities on sedimentary processes and the hydrodynamic environment through sedimentary records can provide a scientific basis for predicting the evolution of the estuary and the sustainable development of the Guangdong–Hongkong–Macao Greater Bay Area. The aims of this study are to reveal the impact of varying intensity human activities across different periods on depositional pattern and conduct a preliminary investigation into the spatial differences in sedimentary characteristic attributed to human activities. Two cores (LD11 and LD13) located in the LE were selected for continuous scanning of high-resolution XRF, grain size, and ²¹⁰Pb_ex dating tests, and scrutinized with the previous studies of the historical process of human activities in the LE. The results show the following: (1) The abrupt alterations in ²¹⁰Pb_ex, geochemical indices, and grain size in LD13 happened in close proximity to the 95 cm layer, suggesting a shift in the sedimentary environment during 1994. (2) In the context of the continuous reduction in water and sediment flux into the LE after 1994, the large-scale and high-intensity human activities like sand mining, channel dredging, and reclamation are responsible for the sedimentation rate increase rather than decrease, the coarsening of sediment fractions, the frequent fluctuations in Zr/Rb, Zr/Al, Sr/Fe, and Sr/Al ratios, and the increase in anomalous extremes. (3) Sedimentary records found in locations varying in anthropogenic intensities differ greatly. Compared with the nearshore siltation area, the grain size composition in the channel area is noticeably coarser and exhibits a wider range of grain size variations. The ²¹⁰Pb_ex is strongly perturbed and the vertical distribution is disturbed; the phenomenon of multiple inversions from the surface downwards is shown, making it impossible to carry out sedimentation rate and dating analysis, and the geochemical indicators have changed drastically without any obvious pattern. The evidence of the human activities can be retrieved in the sedimentary record of the estuary and provide a different angle to examine the impacts of the human activities. Full article

Dredging hoses are flexible and are particularly suitable for slurry transportations for mud or sand in dredging projects. To achieve sufficient bending stiffness and to prevent the pipe body from collapsing, this type of hose segment is a composite structure that is embedded with several cord reinforcement layers and steel wires in its rubber layer. To quickly evaluate the nonlinear bending mechanical properties of rubber hoses, this study proposes the equivalent stiffness method of linear superposition, which is verified by test data and numerical results. The results show that the equivalent bending stiffness method proposed in this study is in good agreement with numerical and experimental results. Then, by comparing the calculation results of the hose string, it was demonstrated that the linear stiffness superposition method proposed in this study can also accurately predict the bending mechanical behavior characteristics of string hose, and provide reliable guidance for hose design in practice. Full article

Sandy beaches high in recreation value make up 16% of the over 4000 km long shoreline of Estonia. The shore processes associated with climate change have remarkably accelerated over recent decades. Many sandy shores have suffered from strong erosion, including an excellent former beach at Valgeranna. The jetties, which were built in the 1860s to protect the navigation channel of Port Pärnu from clogging, have prevented natural sediment transport along the coast from south to north. At the same time, the sandy beach in Pärnu is expanding, and part of the sand accumulates with strong storms also in between the jetties, reducing the width of the shipping channel. The channel needs regular dredging, but, so far, the dredged sediment has been taken far away to the open sea and accumulated on the seabed. The current paper addresses the possibilities of using that sand for beach restoration in destructed and eroded areas. An overview of the applied methods and measurements during field studies is given. The results of modelling the processes of wave activity and sediment transport are discussed. The recycling of shore sediments is an important measure in sustainable coastal zone management. Different options and scenarios are analysed in order to find the most reasonable ways to bring sand back onto beaches and stabilize natural processes. Support from the state by working out respective laws and regulations would be motivating as well. Full article

The morphological changes in an estuarine bay are affected by fluvial and oceanic dynamics, as well as human activities. Human activity has increased considerably in recent years, especially in Lingding Bay of the Pearl River Estuary. Based on mass measured bathymetric data and remote sensing images, morphological changes in Lingding Bay were examined and its long-term morphological evolution from 1964 to 2019 was studied using GIS method and EOF methods. The water area of Lingding Bay gradually decreased through this period due to shore reclamation and the evolutionary characteristics of the underwater topography were different before and after 2007 due to changes in the intensity of human activities. From 1964 to 2007, the water depth and volume of Lingding Bay decreased slightly and the bay experienced a slow silting process with the geomorphic pattern of “three shoals and two troughs” under low-intensity human activity. From 2007 to present, high-intensity sand-dredging activities in the bay have led to considerable deepening and a significant increase in water volume in the East Trough and Middle Shoal areas. The amount of sediment loss caused by the sand-dredging activities after 2007 far exceeded the amount of sediment deposition over the past four decades prior to 2007. Therefore, even if the sand-dredging activities had been banned, the eroded parts of Lingding Bay (i.e., East Trough and Middle Shoal) may not recover in a short time due to the small sediment load from the Pearl River. These recent morphological changes in Lingding Bay may bring about challenges for estuary regulation, disaster control, environmental protection, and the operational safety of the nearby ports and channels. Consequently, the subsequent evolution of the bay requires further research. This will enrich the scientific work for estuarine and coastal research and be conducive to revealing the interaction mechanisms between humans and nature, guiding sustainable development, estuarine disaster control, and promoting interdisciplinary innovation in estuarine research. Full article

Proper orthogonal decomposition (POD) is used to examine the release of highly concentrated water–sediment mixture in water, with or without ambient current. This technique allows us to extract the dominant features in spatio-temporal data sets and the POD total energies associated to the base parameter of the decomposition. Both one-component and two-component POD techniques are, respectively, applied on data relative to the solid volume fraction and on the solid volume fraction velocities. The analysis is based on an experiment in the literature and data sets provided by a two-phase flow solid-fluid numerical simulation. For release phenomenon without ambient current, the analysis of the POD results highlights that the impact of the particle diameter on the solid phase dynamics and the particle dispersion is modest during the falling time, but that it becomes preponderant during the formation of a turbidity current. Aided by POD, the impact of the ambient current is studied for a given particle diameter. As the ambient current becomes strong, we can observe the effect of the resistance of the bottom against the water–sediment mixture transport. According to the strength of the ambient current, the POD results show that the dynamics of the release phenomenon have two different regimes on either side of a clearly identified threshold value. Full article

Dredging materials from reef flats have become an important source of sand and aggregates for meeting the infrastructure needs of coral-lined shores in subtropical and tropical regions, especially for low-lying atoll islands. Dredging at the reef flats can generate artificial excavation pits, which not only have profound influences on coral ecological stability but also deeply affect the hydrodynamic characteristics of coral reefs. To deepen the understanding of the influence of excavation on the wave hydrodynamics of fringing reefs, the wave propagation, wave transformation, wave setup, and wave runup processes of regular waves on fringing reefs with artificial pits have been systematically analyzed using a non-hydrostatic numerical wave solver (NHWAVE). The effects of some significant factors have been carefully investigated. According to the study findings, the existence of artificial pits can result in a slight decrease in the wave height around the artificial pit. The time-mean maximum of wave runup height at the backreef slope can be reduced to some extent when the artificial pit is present. When placed close to the reef edge, the artificial pit can have noticeable effects on the hydrodynamic characteristics of fringing reefs, particularly the wave setup along the reef flat. It is hoped that the study findings can provide further reference for evaluation of the influences of artificial pits on the wave hydrodynamics of fringing reefs. Full article

By collecting surface sediment samples from 158 stations in the near-shore waters of eastern Guangdong, grain size analysis and grain size parameter calculations were performed to explore the characteristics and migration trends of surface sediments in the area. The analysis of the grain size results showed that the surface sediments in the nearshore waters of east Guangdong could be classified into nine sediment types, mainly including seven types of gravel sand ((g)S), gravel muddy sand ((g)mS), gravelly mud ((g)M), sand (S), silty sand (zS), sandy silt (sZ) and silt (Z). The relative percentages of gravel, sand, silt and mud were 0.7%, 40.56%, 46.7% and 12.04%, respectively. The average grain size varied from −2φ to 8φ, with an average of 4.94φ. The selection coefficient ranged from 0.44 to 3.78, with an average value of 1.8. The skewness distribution ranged from −0.34 to 0.67, with an average value of 0.07. By extracting and analyzing the spatial distribution information of grain size in the study area, using the Gao–Collins migration trend analysis method and incorporating dynamic factors such as tidal currents and waves, the transport direction and trend of surface sediments in the study area could be analyzed and inferred. The results show that the surface sediment migration trend was significant, migration on the north side of Nan’ao Island was in an east-to-west direction, and the sediment of Yifeng River was mainly deposited to the sand spout at the mouth of Lianyang River. After southward transport from the Houjiang waterway, the migration was mainly southeastward and the trend was quite significant until the 20 m isobath, where the trend gradually decreased. The sediments of the Rongjiang River were mainly deposited outside the mouth of Niutian Yang and Rongjiang River, and the surface sediments of Guang’ao Bay and Haimen Bay migrated in the northwest–southeast direction. After the 30 m isobath, the southeast corner of the study area migrated in the southeast–south direction. This sediment transport pattern revealed by the grain size migration trend is in good agreement with the physical and hydrodynamic conditions of the study area and provides an important reference for decisions regarding port dredging and waterway management in the area. Full article

As natural aggregates become increasingly scarce, attention has turned to ultra-fine dredged sand (UDS) generated in waterway regulation engineering. UDS is typically challenging to utilize due to its high clay content and high water demand. This article uses ground granulated blast-furnace slag (GGBS)-based geopolymer to solidify UDS, along with sodium silicate (SS) and sodium hydroxide (SH) as alkaline activators. This paper explores the effects of SS modulus (SiO₂/Na₂O molar ratio) and mass percentage content of Na₂O on the fluidity, setting time, mechanical properties, and shrinkage behavior of hybrid UDS-GGBS geopolymer (HUGG) paste. According to the research findings, increased SS modulus and Na₂O content lead to decreased fluidity and setting time. When the Na₂O content reaches 6%, flash coagulation occurs in the slurry, leading to more internal shrinkage cracks and pores. This has been confirmed by scanning electron microscope (SEM) analysis. When Na₂O content is 5%, and SS modulus is 1.0, the 90-day maximum compressive strength reaches 56.53 MPa, corresponding to a splitting tensile strength of 6.83 MPa, which can be considered the optimal formulation. Meanwhile, basalt and polypropylene fibers (BF and PPF) are chosen to compensate for the susceptibility to drying shrinkage. Both BF and PPF can significantly inhibit the linear drying shrinkage of the HUGG paste. The BF’s ability to enhance mechanical properties is more robust than PPF’s, which can make the paste more homogeneous. The research contributes an effective method for the resource utilization of UDS. Full article